Volume 116, Issue 2, August 2004
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
116(2004); http://dx.doi.org/10.1121/1.1766053View Description Hide Description
This paper draws attention to symmetric Lloyd–Redwood (SLR) waves—known in ultrasonics as “squirting” waves—and points out that their distinctive properties make them well-suited for carrying positive feedback between rows of outer hair cells. This could result in standing-wave resonance—in essence a narrow-band cochlear amplifier. Based on known physical properties of the cochlea, such an amplifier can be readily tuned to match the full 10-octave range of human hearing. SLR waves propagate in a thin liquid layer enclosed between two thin compliant plates or a single such plate and a rigid wall, conditions found in the subtectorial space of the cochlea, and rely on the mass of the inter-plate fluid interacting with the stiffness of the plates to provide low phase velocity and high dispersion. The first property means SLR wavelengths can be as short as the distance between rows of outer hair cells, allowing standing wave formation; the second permits wide-range tuning using only an order-of-magnitude variation in cochlear physical properties, most importantly the inter-row spacing. Viscous drag at the two surfaces potentially limits SLR wave propagation at low frequencies, but this can perhaps be overcome by invoking hydrophobic effects.
116(2004); http://dx.doi.org/10.1121/1.1771571View Description Hide Description
Motivated by recent experimental results, an explanation is sought for the asymmetry in the radial profile of basilar membrane vibrations in the inner ear. A sequence of one-dimensional beam models is studied which take into account variations in the bending stiffness of the basilar membrane as well as the potential presence of structural hinges. The results suggest that the main cause of asymmetry is likely to be differences between the boundary conditions at the two extremes of the basilar membrane’s width. This has fundamental implications for more detailed numerical simulations of the entire cochlea.
Relationship of neural and otoacoustic emission thresholds during endocochlear potential development in the gerbil116(2004); http://dx.doi.org/10.1121/1.1771613View Description Hide Description
Distortion product otoacoustic emissions and auditory brainstem responses (ABRs) were measured in neonatal gerbils at three ages: at 15–16 days after birth (dab), near the onset of hearing when the endocochlear potential (EP) is known to be still immature; at 22 dab, when the EP first reaches mature levels; and at 30 dab. Comparing individual 15–16 dab animals to the 22 dab group, ABR threshold changes were typically larger than those for cubic distortion tone (CDT, emission thresholds which were, in turn, larger than those for the simple difference tone (DT, In contrast, from 22 to 30 dab there were no important changes in CDT or DT emission thresholds. Observed threshold-change relationships were very similar to those found in differential diagnosis investigations, where the EP was experimentally decreased using a chronic furosemide application. Therefore, most of the change in cochlear function over the two week period studied could be attributed to the maturation of EP during the first week. Model calculations further show that relative changes in CDT and DT emission thresholds are compatible with a movement of the operating point of the cochlear amplifier toward its symmetrical “central” point as the EP reaches mature levels.
Chronic excitotoxicity in the guinea pig cochlea induces temporary functional deficits without disrupting otoacoustic emissions116(2004); http://dx.doi.org/10.1121/1.1772395View Description Hide Description
Brief cochlear excitotoxicity produces temporary neural swelling and transient deficits in auditory sensitivity; however, the consequences of long-lasting excitotoxic insult have not been tested. Chronic intra-cochlear infusion of the glutamate agonist AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) resulted in functional deficits in the sound-evoked auditory brainstem response, as well as in behavioral measures of hearing. The electrophysiological deficits were similar to those observed following acute infusion of AMPA into the cochlea; however, the concentration-response curve was significantly shifted as a consequence of the slower infusion rate used with chronic cochlear administration. As observed following acute excitotoxic insult, complete functional recovery was evident within 7 days of discontinuing the AMPA infusion. Distortion product otoacoustic emissions were not affected by chronic AMPA infusion, suggesting that trauma to outer hair cells did not contribute to AMPA-induced deficits in acoustic sensitivity. Results from the current experiment address the permanence of deficits induced by chronic (14 day) excitotoxic insult as well as deficits in psychophysicaldetection of longer duration acoustic signals.